In making a lithographic printing plate, a so-called gum solution is coated at the final process.
The gum solution is coated for the purpose of not only protecting the water wettability of the non-image region but also preventing the plate from scumming or flaws caused by the adhering of fingerprint, ink oil or dirt at the image correction such as addition or deletion on the image region, during storage until the plate is used for printing after the plate making or until the plate is again used, at the time of mounting the plate to a press, or during the operation. Furthermore, the gum solution is coated to prevent the generation of oxidation scumming.
Heretofore, an aqueous solution of gum arabic, cellulose gum or a water-soluble polymer material having a carboxyl group within the molecule has been generally used as the gum solution for a lithographic printing plate. These gum solutions have, however, the following problems. That is, in the final process for finishing the printing plate, a gum solution is poured on the plate, extended throughout the plate surface with a sponge or cotton tampon and then rubbed with a cloth until the plate surface is dried. At this time, the thickness of the water-soluble polymer material coated partially remains large in the image region (region of receiving ink). The image area on the large thickness part is poor in the inking property at the printing and a fairly large number of sheets must be printed until a printed matter having a desired ink density can be obtained. In general, this phenomenon is called as dropout printing (so-called inking failure).
For solving this problem, use of a plate surface protective agent for a lithographic printing plate, containing a nonionic surfactant such as polyoxyethylene-polyoxypropylene block copolymer is conventionally known.
On the other hand, in the trend, the environmental safety is a matter of great importance and a highly safe plate surface protective agent for a lithographic printing plate is being demanded. Another important matter is the reduction of the waste solution and in the predominant plate making process, the plate surface protective gum is coated on a developed plate as it is without passing through water washing. Under these circumstances, a novel plate surface protective agent having good coatability and running suitability with good durability is demanded. In particular, a plate surface protective agent having a highly long-term stability against the mixing of a developer component is demanded.
Further, the lithographic printing is a printing system ingeniously using the property such that water and oil are substantially not mingled with each other. The printing plate surface consists of a region of accepting water and repelling oily ink and a region of repelling water and accepting oily ink. The former is a non-image region and the latter is an image region.
The desensitizing agent has an action of enlarging the surface chemical difference between the image region and the non-image region by wetting the non-image region with a fountain solution containing the desensitizing agent.
Heretofore, as the fountain solution, an aqueous solution containing an alkali metal salt or ammonium sat of bichromic acid, a phosphoric acid or a salt thereof such as an ammonium salt, or a colloid substance such as gum arabic or carboxymethyl cellulose (CMC) is commonly known.
However, the fountain solution containing only such a compound disadvantageously has difficulty in uniformly wetting the non-image area of the plate and therefore, suffers from a problem such that the printed matter is sometimes stained or a considerably high skill is necessary for controlling the amount of the fountain solution supplied.
In order to overcome these problems, a Dahlgren system of using an aqueous solution containing from about 20 to 25% of isopropyl alcohol has been proposed. This system is advantageous in many points in view of workability and fidelity of the printed matter. For example, wetting of the non-image area is improved, the amount of the fountain solution can be reduced, the balance in the amount between the printing ink and water supplied is easily controlled, the amount of the fountain solution emulsified into the printing ink decreases and in turn, transferability of the printing ink to a blanket is improved.
However, the isopropyl alcohol readily evaporates and in order to maintain a constant isopropyl alcohol concentration in the fountain solution, a special apparatus is required and this gives rise to increase in the cost. Furthermore, the isopropyl alcohol has a peculiar malodor and also has a problem in the toxicity, accordingly, use thereof is not preferred in view of the working environment. In the case of applying a fountain solution containing an isopropyl alcohol to offset printing where a dampening roller is usually used, the isopropyl alcohol evaporates on the roller or plate surface and disadvantageously fails in exerting the effect.
In recent years, social concern for the industrial pollution is very high and discharge of chromium ion in the waste solution is strictly regulated. There is also a tendency to regulate the use of an organic solvent such as isopropyl alcohol in view of the safe hygiene. Accordingly, a fountain solution free of these substances is being demanded.
In order to satisfy this demand, for example, JP-B-55-25075 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-B-55-19757 and JP-B-58-5797 describe a composition containing various surfactants. However, in using such a composition as the fountain solution, the surfactant concentration must be fairly high. Furthermore, in actual lithographic printing, the ink and water under the ink roll, printing plate or fountain solution supplying roll rotating at a high speed, are vigorously moving, therefore, as a matter of problem, water may adhere to the ink film or ink may diffuse on the surface of water. The combination with a surfactant proposed in the above-described publications is not enough to completely solve these problems. Moreover, the fountain solution containing such a surfactant has a defect such that bubbling readily occurs during pumping or stirring.
On the other hand, U.S. Pat. No. 3,877,372 describes a solution containing a mixture of ethylene glycol monobutyl ether with at least one of hexylene glycol and ethylene glycol. U.S. Pat. No. 4,278,467 describes a fountain solution containing at least one of n-hexoxydiethylene glycol, n-hexoxyethylene glycol, 2-ethyl-1,3-hexanediol, n-butoxyethylene glycol acetate, n-butoxydiethylene glycol acetate and 3-butoxy-2-propanol. JP-A-57-199693 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") describes a fountain solution containing at least one completely water-soluble compound selected from propylene glycol, ethylene glycol, dipropylene glycol, diethylene glycol, hexylene glycol, triethylene glycol, tetraethylene glycol, tripropane glycol and 1,5-pentanediol. These fountain solution compositions are free of isopropyl alcohol and advantageous in view of the safe hygiene, however, still have a problem such that the non-image area of a PS plate comprising an anodized aluminum substrate cannot be completely wetted during printing, the non-image area is scummed particularly at the time of high-speed printing, the half-tone image area fails to have a normal shape, and as a result, the halftone image area suffers from generation of so-called ink spreading due to enlarged and uneven dot shape. Furthermore, 2-ethyl-1,3-hexanediol does not have a sufficiently high solubility in water and is disadvantageous in obtaining a high-concentration concentrated fountain solution or an additive for the fountain solution.